The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads (also called writers and sensors), a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization of the MR element, which in turn causes a change in resistance of the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the GMR sensor varies as a function of the spin-dependent transmission of the conduction electrons between ferromagnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the ferromagnetic and non-magnetic layers and within the ferromagnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer (reference layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the SV sensor (about 120° C.) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. FIG. 1A shows a prior art SV sensor 100 comprising a free layer (free ferromagnetic layer) 110 separated from a pinned layer (pinned ferromagnetic layer) 120 by a-non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 130.
FIG. 1B shows another prior art SV sensor 150 with a flux keepered configuration. The SV sensor 150 is substantially identical to the SV sensor 100 shown in FIG. 1A except for the addition of a keeper layer 152 formed of ferromagnetic material separated from the free layer 110 by a non-magnetic spacer layer 154. The keeper layer 152 provides a flux closure path for the magnetic field from the pinned layer 120 resulting in reduced magnetostatic interaction of the pinned layer 120 with the free layer 110. U.S. Pat. No. 5,508,867 granted to Cain et al. discloses a SV sensor having a flux keepered configuration.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-Pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-Pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1A. This improved exchange coupling increases the stability of the AP-Pinned SV sensor at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer.
Referring to FIG. 2A, an AP-Pinned SV sensor 200 comprises a free layer 210 separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic layer 226 and a second ferromagnetic layer 222 separated by an antiparallel coupling layer (APC) 224 of nonmagnetic material. The two ferromagnetic layers 226, 222 (FM1, and FM2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 227, 223 (arrows pointing out of and into the plane of the paper respectively).
A key requirement for optimal operation of an SV sensor is that the pinned layer should be magnetically saturated perpendicular to the air bearing surface. Lack of magnetic saturation in the pinned layer leads to reduced signal or dynamic range. Factors leading to a loss of saturation include demagnetizing fields at the edge of the pinned layer, magnetic fields from recorded data and from longitudinal biasing regions, current induced fields and the coupling field to the free layer.
Analysis of the magnetic state of pinned layers in small sensors (a few microns or less in width), reveals that due primarily to the presence of large demagnetizing fields at the sensor edges the magnetization is not uniform over the area of the pinned layer. FIG. 2B shows a perspective view of an SV sensor 250. The SV sensor 250 is formed of a sensor stripe 260 having a front edge 270 at the ABS and extending away from the ABS to a rear edge 272. Due to the large demagnetizing fields at the front edge 270 and the rear edge 272 of the sensor stripe 260, the desired perpendicular magnetization direction is achieved only at the center portion 280 of the pinned layer stripe, while the magnetization tends to be curled into a direction parallel to the ABS at the edges of the stripe. The extent of these curled regions is controlled by the magnetic stiffness of the pinned layer.
As mentioned above, prior art AP-Pinned SV sensors use an AFM in order to pin the pinned layer magnetization so that the pinned layers do not move around when the head is reading data from the disk, upon application of external magnetic fields, etc. The AFM layers are typically very thick, on the order of 150-200 Å. Due to the large overall thickness, such sensors are typically not practical for use in applications where a thin head is desirable.
What is therefore needed is an AP-Pinned SV sensor having improved AP pinning, allowing use of a thinner AFM. What is further needed is a way to increase the pinning of AP-Pinned layers.
Another technology which uses AP-Pinned layers is fabrication of magnetic media. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A portion of a conventional longitudinal recording, thin-film, hard disk-type magnetic recording medium 300 commonly employed in computer-related applications is schematically illustrated in FIG. 3 in simplified cross-sectional view, and comprises a substantially rigid, non-magnetic metal substrate 302, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited or otherwise formed on a surface 302 A thereof a plating layer 304, such as of amorphous nickel-phosphorus (Ni—P); one or more polycrystalline underlayers 306, typically of Cr or a Cr-based alloy, a magnetic recording layer 308, comprised of one or more layers of cobalt (Co)-based alloys with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 310, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer 312, e.g., of a perfluoropolyether. Each of layers 304-310 may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and layer 312 is typically deposited by dipping or spraying. Glass substrates are also typically used for longitudinal media. For media on glass substrates, the plated NiP layer 304 is omitted and additional seed layers are added between the substrate surface 302 A and the underlayers 306.
In operation of medium 300, the magnetic layer 308 is locally magnetized by a write transducer, or write head, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 308, the grains of the polycrystalline material at that location are magnetized in the direction of the applied magnetic field. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The magnetization of the recording medium layer 308 can subsequently produce an electrical response in a read transducer, or sensor, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (“SMNR”) of the magnetic media. However, when the bit density of longitudinal media is increased above about 50 Gb/in2, thermal instability of the magnetization is encountered when the necessary reduction in grain size approaches the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, is to increase the crystalline anisotropy in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer.
U.S. Pat. No. 6,280,813 describes a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. In this type of magnetic media, referred to as AFC media, the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, with the result that the net remnant magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This allows Mrt to be lowered as required for high bit density while maintaining the necessary grain volume (V) needed for thermal stability. The ferromagnetic film closer to the head (the master layer) is typically made thicker than the film farther from the head (the slave layer) such that the net Mrt is greater than zero. The interface exchange energy density, J, between the ferromagnetic layers is a key parameter in determining the potential increase in stability. Higher J values allow a thicker slave layer to be used such that the higher V can be obtained for the same Mrt.
In order to achieve optimal performance of AFC media, it is important to have good epitaxial growth of the magnetic layers through the spacer layer with in-plane alignment of the Co-alloy c-axis. Since the lattice constant of Ru is larger than the typical Co-alloys, improved epitaxy across the spacer layer can be achieved using Ru-alloys that reduce the lattice parameter.
Accordingly, there exists a need for improved methodology for providing thermally stable, high areal density magnetic recording media, e.g., longitudinal media, with large interface exchange energy density, J, optimal microstructure and crystallographic orientation (i.e., in-plane alignment of the c-axis), and reduced or optimized lattice mismatch between vertically separated ferromagnetic layers and a non-magnetic spacer layer (such as of a Ru-based material) providing anti-ferromagnetic coupling (AFC) of the ferromagnetic layers, wherein each of the ferromagnetic layers is formed of a ferromagnetic alloy composition similar to compositions conventionally employed in fabricating longitudinal magnetic recording media, which methodology can be implemented at a manufacturing cost compatible with that of conventional manufacturing technologies for forming high areal density magnetic recording media. There also exists a need for improved, high areal density magnetic recording media, e.g., in disk form, which media include at least one pair of anti-ferromagnetically coupled ferromagnetic alloy layers separated by a non-magnetic spacer layer, wherein each of the ferromagnetic layers is formed of a ferromagnetic alloy composition similar to compositions conventionally utilized in longitudinal magnetic recording media (such as Co-based alloys) and the lattice mismatch between each of the ferromagnetic layers and the non-magnetic spacer layer is reduced or optimized, leading to improved thermal stability.
The present invention, therefore, addresses and solves problems attendant upon forming high areal recording density magnetic recording media, e.g., in the form of hard disks, which media utilize anti-ferromagnetic coupling between pairs of ferromagnetic layers for enhancing thermal stability, while providing full compatibility with all aspects of conventional automated manufacturing technology. Moreover, manufacture and implementation of the present invention can be obtained at a cost comparable to that of existing technology.